What changes are you thinking of? It's my impression that the DfT has been well and truly clamping down on any excess traincrew resources.
Well superfluous depots for a start. On GWR there are now way too many depots, all needing sufficient work, and frankly, it isn't working. They are an expensive waste. There is no need for 6 depots between Swansea and Paddington for example. Used to be most Swansea trains where a Swansea driver up to London and back. Now there are driver changes at places like Cardiff, Swindon and Reading on some services. This simply isn't necessary.
Traincrew diagramming in my view needs to revolve around (on GWR long distance to give an example), Plymouth, Bristol and Swansea depots, with other depots fitting in as required where there are gaps. So the Plymouth, Bristol and Swansea to London trains would be driven up and back by a Plymouth, Bristol or Swansea driver. Obviously say the early morning trains out of London and late evening ones back would be covered by Paddington crews, etc. We need to get back to depots doing what they are good at. For example Exeter is perfectly placed for the local Devon branch work, yet at present we have a situation where GWR wants all and sundry at Exeter to drive London trains. Similarly, there is no need for Gloucester depot to be driving Bristol to Taunton, especially when it is currently at the expense of Plymoth drivers doing it (when for Plymouth it would provide a really useful diversionary route, but they had to lose it so that Gloucester could have it!). It's these kind of mad inefficiencies that need looking at that over time could lead to big traincrew savings. Sorry for the long post!